Rear focussing is where you separate the operation of the shutter release from the operation of autofocus. Traditionally (at least on Canon cameras) a half-press of the shutter button would start up autofocus, start up exposure metering, and then to take the photo you would continue to press down all the way and thus fire the shutter. Canon bodies have long had the ability, via custom functions, to have a button on the rear (under the thumb) operate autofocus and metering, leaving the shutter button exclusively for the shutter (although it meters again as well). I used to operate my venerable EOS 1N like this something like 15 years ago! When I got back into photography after a longish break, I bought an EOS 30D that didn’t have this capability - either that or I forgot how I used to work. I trashed my 30D relatively quickly, and even when I finally saw sense and got a 1D Mark II, I continued to use the shutter button in the default mode.  The great advantage of separating the two functions is that you can leave the AF mode in “AI Servo/Continuous Focus)” 100% of the time, and when you have a static subject you can focus, stop focussing and recompose, and then just use the shutter button to actually take the picture. No more needing to change the mode between "One Shot" and "AI Servo". One of my favourite photographers Arthur Morris extols the virtues of this technique here, and offers a far better explanation than I can. I’ve wanted to change for a while, especially now that I have the 800mm f5.6 lens – when using this with a 1.4x teleconverter only the central AF point is available, which is pretty useless for composition – but up until now I’ve always been too scared to make the change as I felt I’d be all confused when a great opportunity came along. So throughout Shetland and Tobago I persisted with what I knew best. Finally, this year, I manned up and made the changes on my two camera bodies – think of it as a New Year’s resolution. Best relearn how to do it now before my trip to Morocco, and there aren’t many great photo opportunities in the UK midwinter. So everything I have posted up this year – Bearded Tits, my children (on the other blog), Fieldfares, Ducks, the local Turaco, Gulls – has been with the new settings. It’s still not completely natural, and a big problem with gloves is that I continually hit the exposure lock button at the same time, but gradually I think I’m getting there. It is certainly a lot more versatile, and though there have been several times when I’ve nearly switched it back, I’ve managed to hold off. Wish that I could be that strict with my diet! Hopefully some sunshine tomorrow and an opportunity to get out and practice some more.